Contemporary engineering design professionals are generally in agreement that it is primarily the horizontal ground vibration motions of an earthquake that are damaging to a building. The majority of structural details in buildings are designed primarily to support vertical loads, and the factors of safety used for gravitational dead and live loads are generally considered sufficient to account for vertical seismic loads. Furthermore, vertical earthquake motions are typically less intense.
Severe earthquake excitations occur in close proximity of moderate earthquakes, and at further distances from major earthquakes. For example, for moderate earthquakes (of Richter magnitudes ranging from 5.5 to 6.6) at locations less than 5 miles from the causative fault surface, the peak horizontal ground accelerations typically have been measured in the range of 50% to 125% g (where g is the acceleration of gravity). At locations ranging from 5 miles to 25 miles from the moderate earthquake source, the peak accelerations typically measure 10% to 50% g. Acceleration data for locations near major earthquakes is limited; however, major earthquakes affect much larger areas, have significantly longer durations, and can have somewhat larger accelerations.
Building code regulations typically specify the magnitude and distribution of the minimum horizontal earthquake forces for which conventional buildings should be designed on a linearly elastic basis. The design strength of buildings to withstand the imposed dynamic earthquake forces in a linear elastic manner is well understood and is implemented in the design by quantifiable standard structural procedures. The specified minimum horizontal forces of the building codes typically correspond to forces induced by an earthquake with peak ground accelerations on the order of 50% g, which corresponds to a relatively minor earthquake ground excitation. To design for the much larger moderate or severe ground shaking excitations on a linearly elastic strength basis would considerably increase the cost of a structure. Therefore, building regulations permit a design based on the minimum horizontal forces, but only when the structure has sufficient ductility to absorb the motions and energies of anticipated moderate and severe ground shaking intensities without life-threatening collapse.
The conventional approach to designing buildings is to design and detail the entire structure to have sufficient ductility and energy-absorbing capacity to absorb the motions and energies of an earthquake. This conventional ductility approach depends on distributing inelastic deformations throughout the structure, and is complicated by the large variations in arrangements of structural configurations and details. The ductility and energy absorbing capacities of a structure involve complex interactions of the structural components and loadings that are difficult to quantify and explicitly design for. These complex interactions can best be utilized by incorporating a balanced structural design with regular structural configurations, and ductile detailing for components and connections. The building's design strength is reduced below the horizontal forces that would be caused by severe earthquake motions, based on the ductility of the building. The reduction of the design strength in proportion to the earthquake forces is the reduction factor, or R factor. The R factor is difficult to quantify, and can usually only be approximately estimated. Damage to the building and its contents are expected for moderate and severe ground shaking, but collapse of the building is avoided.
The ductility approach is based on satisfactory performance of buildings with regular configurations and ductile detailing during past earthquakes. Most building codes explicity exclude applying the minimum seismic forces to design nonconventional buildings. Because of known failure of some building types during prior earthquakes, most building professionals recommend against constructions with asymmetric designs split levels, major discontinuities in structural elements, multi-story open spaces, soft first stories, tilt-up construction methods, excessively perforated shear walls, excessively glazed exterior walls, or incompatible building components and structural elements. The conventional ductility approach is difficult to appropriately implement and quantify for irregular structures. The use of unquantified R factors is not appropriate. Collapse of the building becomes a risk. The proper design of nonconventional buildings involves an individual determination of the ductilities and energy absorbing capacities for the components and total assemblage.
The horizontal forces due to severe earthquake excitations can be 10 to 20 times larger than the minimum horizontal seismic forces required by building codes. For such large discrepancies it is difficult to quantify the adequacy of the R factors, and R factors larger than 3 should be very carefully verified. Furthermore, during severe excitations considerable damage to the structure and to non-structural building components and contents can be anticipated. These could lead to serious consequences for facilities that may be essential for operations after an earthquake (such as hospitals, fire and police stations, communication facilities, and municipal administration centers). For any building there are significant risks of extensive damage and loss of function for extended periods of time, which may lead to large economic losses.
In the base isolation approach, the structure is supported on devices that are specifically designed to absorb the motions and energies of the earthquake impact. Base isolation is a conceptually simple approach which is gaining recognition as an effective protection against earthquakes. Unfortunately, the previously available base isolation systems have been difficult and expensive to incorporate into conventional building construction. Furthermore, the vibration isolation devices that are used to isolate machines and equipment from general vibrations have not been applicable to buildings because they usually have small load capacities, can accommodate only small amplitude motions, include vibration isolation from vertical motions, and often incorporate complex mechanical, hydraulic, or pneumatic support systems.
Base isolation systems for buildings that do not have a restoring force are not adequate. They have a zero frequency response and are susceptible to excessively large displacement. These systems are vulnerable to unrestrained displacements resulting from ground rotations or tilting caused by ground distortions or settlements. Systems incorporating independent springs for the restoring force tend to be complex because of the encumbrance of having to also provide a distinct means for vertical support while permitting the lateral movement. Active systems that incorporate electronic feedback and servo-controlled systems are certainly too complex, not reliable enough, and require excessive maintenance.
Base isolation systems using rubber pad supports have had some limited but successful applications to buildings. Contemporary rubber pads employ thin layers of rubber and steel to increase the vertical stiffness. These rubber pads accommodate lateral displacements through shearing strains in the rubber layers. The lateral stiffness of the rubber pads decreases both with increased vertical load and with increased lateral displacements, constituting inherent instability characteristics. These instability characteristics limit the lateral displacements that can be accommodated. The lateral stiffness characteristics of the pad support system are such that eccentricities are expected to occur between the center of lateral resistance and the building's center of mass, inducing torsional response motions. The torsional motions can double the required displacements and strains which the isolator pads must absorb, and it can be difficult to accommodate the required strains without exceeding the stability limit of pads with practical proportions. Rubber bearings with sufficient height to accommodate large lateral displacements have reduced stability and vertical stiffnesses. Reduced vertical stiffness can result in a rocking mode which has a period susceptible to amplification, and can also increase the vertical mode period to a more susceptible range. Local rotations of the connection plates of the pads add to the strains and instability of the pad. These rotations and the lateral displacement instability are controlled by incorporating both a rigid structural framework above the pads and perimeter foundation walls, but this has considerably increased the cost of using such systems.
Another category of base isolation systems which has had some limited but successful applications is that of roller or rocker bearings. Many variations for roller and rocker bearing systems have been proposed. In general, the systems that have restoring forces have worked, but have practical limitations, including: the carrying capacity of each roller bearing is limited by the small contact bearing area; they are awkward and expensive to implement; and they require separate additional energy absorbing mechanisms.
A category of base isolation devices which has received little recognition and attention are pendulum systems. Some systems of this kind for buildings consist of cradle frames and slings with releasable mechanisms to restrain against small amplitude motions. The slings act as pendulum arms, providing the lateral motion capability. However, the cradle frames, slings, and releasable mechanism are cumbersome, difficult to implement, of limited load carrying capacity, and of questionable reliability.
Another proposed column support for buildings includes a pedestal suspended by hanger-rods. The system would not work effectively as proposed because the hanger-rod lengths were proportioned considerably too short, and there is no explicit damping. The system is not practical because it does not easily accommodate a correctly-proportioned pendulum length and swing, has low load-carrying capacity, and is expensive to fabricate and cumbersome to implement.
Another support system includes an earthquake protective platform for electrical apparatus, suspended from rigid pendulum links, and with attached viscous dampers. The system had a predictable period of motion that could effectively be used as a base isolation system. The suspended platform approach, however, is not practical for buildings. Furthermore, the required length of the pendulum links is generally 4 ft (1.2 m) or longer, which creates practical difficulties for application to buildings.
The prior known pivoted sliding supports have not been designed in a manner suitable for base isolation. They have pivot points substantially above the sliding surface, have low load capacities, and do not include a means of achieving reliable hysteretic friction damping. One construction of this type includes a three-point foundation system employing combinations of fixed and sliding supports. This unusual foundation system was designed to accommodate vertical undulating deformations of the ground surface and fissures of the ground surface beneath a building. Two embodiments of the sliding mechanisms employ pivoted shoes on concave surfaces that are used to accommodate relative horizontal ground distortions between the supports, and would appear to work satisfactorily for this purpose. In one embodiment, a fixed support is used in conjunction with the sliding shoes, and the fixed support absorbs the lateral forces and provides lateral stability to the shoe supports. In an alternative embodiment, the construction includes rubber-like bushings beneath the sliding support which would absorb high-frequency small-amplitude motions. If the rubber bushings are designed to protect the sliding shoes from the major lateral inertial forces, then the rubber bushing would be serving as the primary base isolation means.
If the pivoted shoes were used under all supports without the fixed support and without adequate rubber bushings, serious difficulties would arise. The shoes would be directly subjected to the large inertial forces and the high velocity and displacement motions of the horizontal earthquake excitation. Because of the height of the pivot point above the sliding surface, the shoe is subjected to an overturning moment which is equal to the product of the lateral force on the building times the height above the surface. This overturning moment tends to topple the shoe, leading to an instability, and at best irregular sliding motions.
The pivoted shoe designs are such that the heights of the pivot points above the sliding surfaces range from 17% to 33% of the radius of curvature of the sliding surface. Furthermore, when the shoe is in a tilted position the height of the pivot point causes the resultant vector of the weight of the building, which is at the pivot point, to shift toward one edge of the shoe. This shifting of the weight reduces the stabilizing moment provided by the weight of the building and also induces the weighted edge of the shoe to gouge into the supporting surface. The gouging increases the frictional resistance and further contributes to toppling the shoe.
The nonuniformity of the normal pressure also leads to a stick slip phenomenon in the sliding motions. Additionally, if subjected to high-velocity non-lubricated sliding, the surfaces could seize to one another due to a cold welding phenomenon. Most important, sliding the surfaces of the shoe and concave surface would tend to adhere to each other after years of high-pressure contact, and therefore would not slide when required.
At a severe intensity of lateral shaking the supported building may also undergo a lateral rocking motion. This rocking motion would cause a temporary uplift of individual shoes. It is noted that the shoe itself, after uplift, is also subject to horizontal acceleration motions which will rotate the shoe relative to the building. Consequently, the shoe rotates out from under the building, leading to improper alignment and an unacceptable instability upon recontact with the sliding surface.
These limitations apply to any pivoted sliding support where the pivot point is a substantial distance above the sliding surface. For non-lubricated systems these limitations are exacerbated when inappropriate materials for the frictional interface are used. For lubricated systems it is difficult to maintain adequate interface lubrication during prolonged periods of non-sliding. The height of the pivot point above the sliding surface also induces horizontal and vertical displacements of the pivot point and supported building relative to the shoe's sliding surface. The building, therefore, does not follow the same horizontal-vertical kinematic relationship as that of the concave surface.